1. Field of the Invention
The present invention relates to a flexible metal-clad laminate and a flexible, rigid metal-clad laminate using a low-dielectric constant polyimide having a good heat resistance and a good adhesion property. The metal-clad laminate is applicable to printed wiring boards, surface heating elements, electromagnetic shielding materials, flat cables, etc.
2. Description of the Prior Art
Metal-clad laminates have been mainly used as a substrate for printed wiring boards. With the recent rapid progress of miniaturization and high integration of electronic devices using printed wiring boards, there is an increasing demand for high-speed operation of these devices. To meet the demand, flexible metal-clad laminates, the electric insulating layer thereof being capable of being readily reduced in its thickness, have been increasingly used. In particular, in the application fields such as information processing and telecommunications, transmission frequency or operating frequency of CPU becomes higher in order to transmit or process a large content of information data. Therefore, it is required to shorten the time delay of signal transmission rate by reducing the dielectric constant of insulating layer in addition to making the insulating layer thinner.
Aromatic polyimides having excellent heat resistance and mechanical properties have been extensively used as the electric insulating material for the flexible metal-clad laminates. For example, Japanese Patent Application Laid-Open No. 55-91895 proposes a three-layered metal foil-clad laminate comprising an aromatic polyimide film and a metal foil which are bonded to each other through an adhesive film. Japanese Patent Application Laid-Open No. 56-139953 discloses a two-layered metal foil-clad laminate comprising a heat-fusible aromatic polyimide film and a metal foil which are directly press-bonded to each other under heating. However, since epoxy-based or acryl-based adhesives are mainly used as the adhesive in the conventional three-layered flexible metal-clad laminates, the laminates obtained after bonding through such adhesives are insufficient in heat resistance, resulting in limitations to the conditions for subsequent processing and use. The conventional two-layered flexible metal foil-clad laminates exhibit a good heat resistance because the insulating layer is entirely made of polyimide. However, since the polyimide used as the insulating layer is aromatic, the dielectric constant at 10 GHz of the insulating layer unfavorably becomes as high as about 3.5, although varies depending upon the content of aromatic ring.
Japanese Patent Application Laid-Open No. 8-250860 discloses a metal foil-clad laminate obtained by applying an aromatic polyamic acid solution onto a metal foil and then heat-treating the applied solution to imidate the aromatic polyamic acid. However, the aromatic polyimides used in the insulating layers of the conventional flexible metal foil-clad laminates have a dielectric constant as high as about 3.5 at 10 GHz, although it varies depending upon the content of aromatic ring. Further, the aromatic polyimides cannot be made into solutions because of its insolubility to various solvents. Therefore, as described above, the treatment for imidation is required after applying the solution of polyamic acid as a precursor of polyimide in the production of the metal foil-clad laminates, increasing the production facility costs and production costs.
It has been also required to make the wirings of printed wiring boards finer. Therefore, there is a demand for further reducing the thickness of metal foil in some applications. Under this circumstance, there have been developed methods for producing a flexible laminate clad with a metal thin film having a thickness of 10 μm or less which is formed on a surface of an electric insulating film by sputtering, vapor deposition, electroless plating, electroplating or the like (Japanese Patent Application Laid-Open Nos. 9-51163, 8-34866 and 5-90737). Aromatic polyimide films have been extensively used as the electric insulating material for such flexible metal foil-clad laminates. However, as described above, the aromatic polyimides unfavorably have a slightly high dielectric constant, i.e., about 3.5 at 10 GHz although it varies depending upon the content of aromatic ring.
It has been known that the dielectric constant of a polymeric material can be generally reduced by using an aliphatic monomer. The inventors have produced various polyimides from non-aromatic tetracarboxylic dianhydrides. Examples of the non-aromatic tetracarboxylic dianhydride include aliphatic (chain) tetracarboxylic dianhydrides such as 1,2,3,4-butanetetracarboxylic dianhydride, and alicyclic tetracarboxylic anhydrides such as 1,2,3,4-cyclobutanetetracarboxylic dianhydride, 1,2,4,5-cyclopentanetetracarboxylic dianhydride and bicyclo[2.2.2]octa-7-ene-2,3,5,6-tetracarboxylic dianhydride. However, polyimides obtained from the aliphatic (chain) tetracarboxylic dianhydride have an extremely low heat resistance and are inapplicable to the processing such as welding, reducing its practical utility. On the other hand, polyimides obtained from the alicyclic tetracarboxylic anhydride have an improved heat resistance as compared to those obtained from the chain tetracarboxylic dianhydride. However, since polyimides obtained from 1,2,3,4-cyclobutanetetracarboxylic dianhydride have a low solubility to solvents, the application of a solution of such polyimides onto a metal foil or a film forming substrate fails to form a polyimide film having a sufficient thickness. Although 1,2,4,5-cyclopentanetetracarboxylic dianhydride and bicyclo[2.2.2]octa-7-ene-2,3,5,6-tetracarboxylic dianhydride provide polyimides having a high solubility to solvents, the laminate obtained by applying the polyimide solution onto a metal foil or a film-forming substrate fails to have a flexibility, reducing the practical utility.